Door knob



FIG. 1 is a perspective view of the new design for a door knob;

FIG. 2 is a front elevational view of the new design for a door knob;

FIG. 3 is a rear elevational view of the new design for a door knob; and,

FIG. 4 is a right side elevational view of the new design for a door knob, the left side elevational view being a mirror image.

The broken lines in FIG. 3 are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a door knob, as shown and described. 